In a spread spectrum spectrum communication system, it is essential that it can obtain an appropriate threshold value according to any change in the correlator output so as to never fail to detect a required correlation output.
A prior art system is shown, for example, in Japanese Post-Examination Publication No. JP-P-60-5639B entitled "Receiving Circuit in a Spread Spectrum Communication System".
This system is arranged so that peaks of positive and negative correlated spikes of an output of a matched filter are held respectively by a peak hold circuit and are subsequently added. A threshold circuit which generates a threshold value proportional to the resulting peak hold value permits correlation spikes to pass therethrough to perform data demodulation. A circuit arrangement thereof is shown in FIG. 3 where reference numeral 21 refers to a correlator, 22 to a peak hold circuit, 23 to a computing circuit, 24 to a flip-flop, 25 to a shift clock generating circuit, 26 to a shift circuit, 27 to a PN code, 28 to a delay circuit, and 29 and 30 to multipliers which behave here as inverters by multiplying -1. That is, a peak hold circuit 31 holds a positive peak whereas a peak hold circuit holds a negative peak. A threshold is obtained from such a peak value via a variable resistor R3 to use the threshold value to detect a positive correlation spike in a comparator 33 and detect a negative correlation spike in a comparator 34.
The prior art circuit arrangement, however, involves the following problems. In order that the peak hold circuit 22 completely holds the peak of a correlation spike, the interior resistance of a diode D1 or D2 and the time constant of a capacitor C1 or C2 must be very small because the correlation spike width is very narrow. That is, the charge time constant must be small.
In contrast, in case of holding the peak value for a time corresponding to about one period of the correlation spike, the time constant defined by a resistor R or R2 and the capacitor C1 or C2 must be large in order to prevent a decrease in the hold value which is called "droop". That is, the discharge time constant must be large.
Referring to the circuit arrangement of FIG. 3, in order to establish a threshold which is variable in response to changes in the correlation spike .phi. (t), the discharge time constant R1C1 or R2C2 of the peak hold circuit must be large as apparent from FIG. 4.
When considering the follow-up property to change in the peak hold value, a peak hold circuit having an excellent hold property, i.e., a large discharge time constant exhibits a poor follow-up property to decrease in the peak value. This is explained below, referring to FIG. 5.
When a correlation spike .phi. (t) (in this case, data corresponds to 1,1,0,0) which exhibits level changes shown in FIG. 5 is entered in the peak hold circuit 22, values of the peak hold circuits 31 and 32 are S.sub.A and S.sub.B in (b) and (c).
If a correlation spike 2 smaller than a negative correlation spike 1 or a correlation spike 4 larger than a negative correlation spike 3 is obtained, the capacitor C1 or C2 is not changed and continues to discharge. That is, in the event that the peak value is decreased more than the droop caused by the discharge, the peak value cannot be detected. Further, when a threshold S.sub.C and a threshold S.sub.D obtained by multiplying the threshold value S.sub.C by -1 in the multiplier 30 are as shown in FIG. 5a), correlation spike 1 alone is detected, and correlation spikes 2, 3 and 4 are not detected.
As a result, a demodulated data d(t) is an erroneous data with respect to the inputted data. In FIG. 5, (a) and (e) indicate waveforms of S.sub.E and d(t) of FIG. 3 respectively.
That is, when the correlation spike .phi.(t) varies as shown in FIG. 5, it is difficult to detect correlation spikes as far as fixed positive and negative thresholds S.sub.C and S.sub.D are used.
Further, as shown in FIG. 6, also when the received signal level is not changed, it is sometimes difficult to detect correlation spikes using threshold values S.sub.C and S.sub.D obtained through the peak hold circuit 22, because of natures of employed circuits and elements, e.g. a level difference between positive and negative correlation spikes (in FIG. 6, the positive correlation spike is always larger than the negative correlation spike).